Electric arc furnaces are used in the production of steel from scrap iron and other metals. The electric arc furnaces typically include a substantially cylindrically configured melting vessel having an upper shell that defines a top opening. Water-cooled panels form the inside surface of the upper shell. A water-cooled roof assembly is supported by the upper shell and positioned over the top opening. A lower shell includes a refractory brake lining, or the like, where the molten steel is melted by means of at least one electrode, and typically three electrodes in an AC furnace. The electrodes are mounted in the central refractory located in the water-cooled roof assembly and extend into the melting vessel to provide the electrical current to melt the scrap and provide the heat necessary for a steel melt.
A slag discharge opening can be formed under a water-cooled panel to allow discharge of the slag. The steel melt is discharged through an opening in the melting vessel when the melt (or heat) is complete.
The water-cooled roof assembly protects the structure against the high temperatures of the furnace and works in conjunction with the water-cooled panels formed on the inside surface of the melting vessel. In a standard design shown in the prior art of FIG. 1, a plurality of generally pie-shaped water-cooled panels are made from pipes and mounted between an outer ring and an inner ring. The "delta area" is formed from a refractory material and positioned in the central opening. Typically in an AC electric arc furnace, three electrodes extend through the refractory and into the furnace. A small exhaust opening is formed in one of the pie-shaped water-cooled panels to allow exhaust from operation of the electric arc furnace. The number of panels typically varies from 4-10 panels, and in the prior art of FIG. 1, four pie-shaped panels are illustrated.
This type of conventional water-cooled roof has been beneficial. When a pie-shaped water-cooled panel fails, the panel can be replaced with another spare water-cooled panel. However, this convenience has a price because the initial cost of a conventional roof, as illustrated in the prior art of FIG. 1, is greater because a more complicated roof design is necessary to hold the panels. Also, individual pie-shaped panels cost more. There is also an additional cost of a flexible hose piping system to supply and return water to each pie-shaped panel. The flexible hose connections increase the costs and increase the time in replacing an individual panel. The conventional/standard design as shown in prior art FIG. 1 also has pressure losses across the roof that are usually greater than the pressure losses found in a monolithic roof design, where there are fewer elbows.
A conventional monolithic roof design is shown in prior art FIG. 2, where only one water-cooled panel is formed by the roof and includes a central opening as in the prior art of FIG. 1. The monolithic roof design is initially lower in cost and is designed to include a larger water-cooled panel with multiple circuits formed in the shape of the roof. Usually the cooling pipes are rolled into a circle. The radius of each cooling pipe is smaller as the center of the roof is approached. A common header supplies and returns the cooling water. However, this one-piece roof, often referred to as a monolithic roof, does not permit economic replacement when cooling pipe damage has occurred. The roof must be repaired where the damage has occurred. When the damage is great, the roof will have to be removed and repaired elsewhere or, in a worse case scenario, scrapped, and a new roof purchased.
Typically, in both monolithic and in conventional water-cooled roofs, higher stresses and greater failure rates occur at the center of a roof section where the first few pipes closest to the refractory center are located. Thus, in a standard roof design, a pie-shaped panel will typically have only a small portion of a cooling pipe damaged by heat and stress. If an entire pie-shaped water-cooled panel is removed, it must be replaced with a new water-cooled panel, which increases the maintenance costs and wastes materials.